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1
To prepare cookies:.
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Preheat oven to 400 degrees and line baking sheets with parchment paper.
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3
In a bowl of an electric mixer on medium-high speed, beat butter and sugar until light and fluffy, about 2 minutes.
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4
Add egg and lemon juice and mix until thoroughly combined.
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Reduce speed to low, add flour and salt and mix until incorporated (if dough is too stiff to pipe, add an additional 1/4 teaspoon lemon juice or beaten egg).
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6
Transfer dough to a pastry bag fitted with a #21 round tip.
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7
On prepared baking sheets, press dough into a 1-inch wide coil, building up two layers of dough.
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8
Repeat with remaining dough, placing cookies 2 inches apart.
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9
Bake until cookies are set but not browned, 6 to 8 minutes.
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10
Remove from oven and quickly make indentations in centers of cookies with the end of a wooden spoon.
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11
Cool cookies on baking sheets for 2 minutes, then transfer cookies to a wire rack to cool completely.
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12
To prepare lemon curd:.
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13
In a double boiler over gently simmering water, combine egg and sugar, whisking continually.
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Add butter, lemon juice, lemon zest and salt and continue whisking until curd is thickened and has reached 160 degrees.
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15
Remove from heat.
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16
Using a small spoon, fill centers of cookies with lemon curd.
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17
Cover and refrigerate up to 1 week.
